The present invention relates to a method and apparatus for the production of spacecraft-grade nitrogen tetroxide (N.sub.2 O.sub.4) for use as a bipropellant rocket engine and, more particularly, to a method and portable apparatus which removes water, iron ions and chloride ions with suitable molecular sieves and utilizes catalytic oxidation of ammonia in a gas phase.
Current military spacecraft missions are using nitrogen tetroxide and monomethyl hydrazine (N.sub.2 H.sub.CH.sub.3) as the oxidizer and propellant, respectively, for bipropellant rocket engines. Current and planned military space systems project increased use of the bipropellant systems, replacing older hydrazine engines. Such bipropellant systems provide more energy per unit propellant mass than monopropellant systems. Unfortunately, current N.sub.2 O.sub.4 being supplied to the military and produced by oxidation of nitrosyl chloride causes clogging of filters, valves and nozzles in the propulsion systems due to undesirable levels of dissolved iron, chlorides, water and non-volatile particulates in the oxidizer. Such clogging can thus cause the unnecessary firing of propellant engines to clear the clogs or can render the propulsion system useless, resulting in the loss of spacecraft.
The production of nitrogen tetroxide for industrial purposes has been well known for a long time. For instance, U.S. Pat. No. 3,101,255 describes the production of N.sub.2 O.sub.4 by the oxidation of ammonia as an alternative to the preparation of nitric acid, since it is easier and more economical to store and ship N.sub.2 O.sub.4 than nitric acid. The objective was to produce N.sub.2 O.sub.4 in a normal level of purity without the formation of dilute nitric acid or other by-products. Ammonia was reacted in the presence of stoichiometric amounts of oxygen diluted with nitrogen. In practice, there was a slight excess, 1%, of ammonia. The conventional process removed water before oxidizing the NO to NO.sub.2 with stoichiometric portions of oxygen. The NO.sub.2 polymerized to gaseous N.sub.2 O.sub.4 which was condensed to a liquid. The mixture of air, nitrogen, and ammonia was adjusted to give a maximum concentration of oxygen of 12%. Gases were cooled to a temperature of 50.degree. F. and dehumidified before the NO was oxidized. The pressure of the gases in the system was between 45 and 170 psig. For the first oxidation step and after dehydration, the NO was passed through silica gel for additional water removal before final oxidation to N.sub.2 O.sub.4. Nitrogen was recycled to dilute the incoming air. The reaction temperature reached 1700.degree. F. during the initial oxidation. This heat was transferred to the incoming air with a heat exchanger before it was cooled to 50.degree. F. Nitrogen from the heat exchanger cooling was used as the incoming nitrogen. A waste heat boiler was used to collect heat from the initial oxidation step following by mechanical refrigeration to cool the gas stream to 50.degree. F. The water vapor was first separated mechanically and then by adsorption. Cooling water was used in the second step.
The known process used platinum or platinum alloys as the catalyst for the oxidation. The temperature of the preheated air was 500 to 540.degree. F. at the entrance to the converter. There was a small amount of hydrogen produced in the process for the decomposition of ammonia. Dual adsorption beds were continuously regenerated and used to remove the last traces of water from the N.sub.2 O.sub.4. The temperature after the NO oxidation process had to be below 73.degree. F. for the N.sub.2 O.sub.4 to form.
The above-referenced patent does not refer to the purity or level of water present in the final product. Given the state of the art and the industrial applications existing at that time, however, it can only be assumed that the known process produced N.sub.2 O.sub.4 at a normal level of purity, particularly since there is no discussion of construction materials which we have found necessary for producing high purity N.sub.2 O.sub.4. The known process does not teach the art how to produce very high purity N.sub.2 O.sub.4 for today's specific applications such as spacecraft propellants.
U.S. Pat. No. 3,070,425 does describe a process to produce N.sub.2 O.sub.4 with a minimum purity of 99.5%. Water is used as the coolant without the aid of refrigerant. In this known process, a mixture of NO, NO.sub.2, oxygen, and water vapor are diluted in nitrogen, and then the NO is oxidized to NO.sub.2 and the water is removed by condensation. The purity of NO.sub.2 is increased first by its adsorption in concentrated nitric acid (85 to 95%) and then distilling the NO.sub.2 from the concentrated nitric acid. The NO.sub.2 fraction is condensed to produce N.sub.2 O.sub.4 as the temperature is lowered. Conversion of ammonia to NO was accomplished on either a platinum, platinum-rhodium, or cobalt-nickel catalyst at pressures up to 100 psig. The reaction temperatures were in the range of 800 to 960.degree. C. Stainless steel was used as the material of construction for the condenser. Stoneware was used as the packing in the nitric acid adsorption columns and oxygen was added to the N.sub.2 O.sub.4 water mixture to increase the yield of HNO.sub.3 by the reaction: EQU N.sub.2 O.sub.4 +1/2O.sub.2 +H.sub.2 O.fwdarw.2 HNO.sub.3
There was also a loss of NO.sub.2 by reaction with H.sub.2 O to form HNO.sub.3 and NO.
Fractionalization of the concentrated nitric acid was done in a multi-plate column constructed of stainless steel, titanium, and tantalum with a glass lined inner tube. The final product had a purity of 99.5% and the product specifications are given below:
______________________________________ N.sub.2 O.sub.4 99.5% minimum H.sub.2 O 0.1% maximum (1000 ppm) Cl as NOCL 0.08% maximum (800 ppm) Non-volatiles 0.01% maximum (100 ppm) ______________________________________
These specifications with regard to water, Cl and non-volatiles are many times higher than those needed for spacecraft grade N.sub.2 O.sub.4. Although this patent describes a method for preparation of 99.5% minimum purity N.sub.2 O.sub.4, such a method would not produce a sufficiently high purity for today's high technology applications in a simple manner which permits the use of portable and self-contained apparatus.
U.S. Pat. No. 3,063,804 describes a process where high purity N.sub.2 O.sub.4 was produced by rapidly cooling a mixture of NO to a temperature above its dew point (approximately 90.degree. C.) in a reactor with countercurrent flow to relatively cool nitric acid (concentration 60 to 70%) to produce higher oxides of nitrogen. Then water and nitric acid were removed as condensate and the overhead was primarily NO.sub.2, N.sub.2, and O.sub.2. Residual water and nitric acid were removed by fractional distillation of crude N.sub.2 O.sub.4 to give a final product purity of 99+% nitrogen tetroxide. The process used the liquid phase reaction: EQU NO+2 HNO.sub.3 .fwdarw.3 NO.sub.2 +H.sub.2 O
for the conversion of NO to NO.sub.2, which is favored at high acid concentrations and high temperatures. Again, however, such a process did not produce a product of sufficient purity with a portable and self-contained apparatus using a simple method which avoids waste by-products and allows the product to move freely through the production apparatus.
It is, therefore, an object of the present invention to produce a spacecraft-grade N.sub.2 O.sub.4 with a portable and self-contained apparatus and method which filters the product and avoids waste by-products.
We have found that the impurity problem of commercially available N.sub.2 O.sub.4 can be overcome by producing the oxidizer via catalytic combustion of ammonia in a gas phase. Catalytic combustion of ammonia yields inherently pure N.sub.2 O.sub.4.
According to a presently contemplated apparatus and method for carrying out the present invention, commercial grades of ammonia and air (a flow ratio of about 10% ammonia) are passed to an ammonia converter that contains a conventional platinum screen catalyst. The ammonia combustion is initiated by preheating the catalyst screen using a hydrogen flame, which is shut off after ignition or heating the catalyst screen with hot air to ignite the ammonia. The combustion of ammonia produces nitric oxide and water, which are passed through two condenser/coolers that collect the majority of the water by-product. The product gases are then mixed with pure oxygen to oxidize the nitric oxide product to nitrogen dioxide. The gaseous nitrogen dioxide is then passed through a desiccant such as silica gel or molecular sieves to perform gas-phase removal of the remaining low-level water and any iron entrained in the product. The nitrogen tetroxide is collected by freezing the product in a collection cylinder.
The various reactions of the process are: EQU 4 NH.sub.3 +5 O.sub.2 .fwdarw.4 NO+6 H.sub.2 O EQU 4 NO+2 O.sub.2 .fwdarw.4 NO.sub.2 EQU 4 NO.sub.2 .fwdarw.2 N.sub.2 O.sub.4
Other competing side-reactions could take place, but the above-specified reactions are the primary process reactions.
The results of analysis of the final product have demonstrated that the present invention is advantageous in that it has produced N.sub.2 O.sub.4 which has exceeded the five key product specifications of (1) dissolved chloride, (2) dissolved iron, (3) nitric oxide content, (4) water content and (5) nitrogen tetroxide purity. In addition, the present invention has the advantages of providing the ability to control the NO content of the product by adjusting various process parameters of the process.
One presently contemplated embodiment of the apparatus of the present invention comprises an air preheater, an ammonia/air converter, a plurality of cooler/condensers, a desiccant such as a plurality of molecular sieve or silica gel columns, or a combination thereof, and a product collection cold finger. The apparatus is constructed completely of 304 or 316 stainless steel (e.g. 304 or 316 grade) and glass. All stainless steel components are fastened to an angle iron framework assembled in a fume hood. Alternatively, all glass components are clamped to ring stands in a fume hood. Glassware connections are made with ground-glass joints or with short sections of Teflon.RTM. tubing.
Raw materials for the process can consist of medical breathing air which is a mixture of liquid nitrogen and liquid oxygen, anhydrous ammonia, and industrial grades of hydrogen and oxygen. Appropriate and conventional gas regulators and check valves are used with all gases. Flow rates of air, ammonia and oxygen are monitored using standard flowmeters. The air and ammonia are connected to the converter via stainless steel tubing, e.g. 1/4" OD tubing. The oxygen is delivered downstream to the apparatus via Teflon.RTM. tubing, whereas the hydrogen is delivered to the converter with stainless steel tubing.